Much of the research and development effort in the field of automotive finishes is currently directed to the development of color coat/clear coat paint systems. It has been found that an excellent appearance, with depth of color and with metallic glamour, can be obtained by applying a clear coat over a pigmented coat. Minimal visual color of clearcoats is considered by users to be an important attribute of color coat/clear coat paint systems. Unfortunately, the excessive yellowness of many automotive clear coats in the can as well as on the substrate has always been a problem which has never been solved successfully.
The most obvious color problems resulting from having a clear coat which is amber or yellow occur when the clear coat is used over white color coats. This results in a final color which is either too creamy or perhaps light tan instead of the desired white. Other less obvious color problems occur on silver color coats, where the final color becomes too gold, or on light blue color coats, where the result becomes greenish. These color problems are more apparent away from fluorescent light, which has blue wavelengths that cancel some of the yellow tones. Unfortunately, outdoor light has what artists call a "warmer" tone, rich in red and yellow wavelengths, that tends to bring out a yellow cast in a clearcoat job. The color problem is also accentuated at higher film builds or when viewed from a grazing angle (sidetone) since the light has more clearcoat to travel through. Prior to the use of base coat/clear coat systems for the automotive industry the issue of excessive color in clear coats was not as important because the film thickness of the clear was minimal. Current clear film builds are 1.5 to 2.0 mils or more. In the past the clear film builds were typically less than 0.5 mils.
Our novel method of adding small amounts of blue or violet pigment to the clear creates an unprecedented level of clarity in the clear coat and eliminates all the disadvantages of the yellowing clears described above.
The use of pigment in a clear coat used over a base coat has been tried before. However, our reasons for adding the pigment to the clear, and the ratio of pigment to binder are completely different. For instance, in the past pigment has been added to clear for the following reasons: (1) using the pigment in the clear as an ultraviolet light absorber to help protect the coating; (2) using the pigment in the clear to supplement the color in the base coat; and (3) using the pigment in the clear for the aesthetic effect intrinsic to the pigment used (i.e. actually coloring the clear rather than using it for chroma neutralization). Our invention is novel in its use of pigment in the clear to neutralize the chroma intrinsic in many clear products, such as the yellowness of many polymeric clears. Below are brief summaries of the patents in the prior art.
The Panush patents (U.S. Pat. Nos. 4,449,143; 4,598,015 and EP No. 169-796) are specific to encapsulated mica pigments. EP No. 148-718 discloses a broader range of pigments but has a relatively high pigment to binder ratio of 0.2/100. The purpose of the pigmentation in all these patents is to actually produce color effect in the clear, or to protect the paint system from ultraviolet light.
The Meyer (U.S. Pat. No. 4,168,249) and Walus (U.S. Pat. No. 4,273,690) patents disclose basic resin compositions that could be used for clears. They further disclose adding pigments to these resins to make paint in a conventional manner. The pigments are to be used at a relatively high pigment to binder ratio of at least 1/100. The purpose of pigmentation is not for chroma neutralization.
The Chang patents (U.S. Pat. Nos. 4,208,465 and 4,355,071) specifically claim antioxidant and ultraviolet light stabilizers and inorganic siliceous pigments of refractive index 1.4-1.6. The pigments are added to protect the paint from degradation, not for chroma nuetralization.
The Fry et al patent (U.S. Pat. No. 4,389,502) claims adding pigments for mechanical or rheological properties. The pigment must have a refractive index similar to that of the film forming constituents used in the clear. Other pigmentation is claimed only when the composition is employed other than as a clearcoat of a clearcoat/color coat finish and the minimum pigment to binder ratio is 1/100. This also is true of the Craven patent (U.S. Pat. No. 4,402,983).
The DeBergalis patents (U.S. Pat. Nos. 4,522,971; 4,577,007; 4,585,693 and 4,636,431) claim compositions which have pigments which are highly ultraviolet light absorbing.
Patent JA No. 53055348 assigned to Kansai claims pigmentation at a pigment to binder ratio of at least 1:100. At these pigment to binder ratios you no longer have a clear coat.
Patent JA No. 56115664 assigned to Dainippon claims use of non-yellowing type polyisocyanates but does not claim pigmentation.